The invention relates to open-cell moldable foams based on styrene polymers.
An advantage of open-cell foams is that they can be evacuated. The resultant evacuated vacuum moldings have considerably lower thermal conductivity and are therefore significantly better insulators than conventional foams.
Open-cell extruded foam sheets and open-cell extruded foam boards based on styrene polymers are known, e.g. from EP-A 642,907, WO 9600258 and WO 96/34038. They are produced by extruding a polystyrene melt to which a volatile blowing agent and a nucleating agent have been added, at temperatures of from 110 to 140xc2x0 C. In WO 98/58991, from 0.1 to 7% by weight of an ethylene-vinyl acetate copolymer is to be added to the polystyrene during the process.
However, foam sheets and foam boards are of simple shape. Their thickness and width is predetermined by the dimensions of the extrusion equipment.
Polystyrene foam moldings of any desired dimensions and shape may be produced in a known manner by foaming polystyrene pellets comprising blowing agents and sintering the resultant moldable foam.
It is an object of the present invention to provide open-cell moldable foams based on styrene polymers.
We have found that this object is achieved if the thermoplastic matrix comprises A. from 99 to 50% by weight of a styrene polymer and B. from 1 to 50% by weight of a polymer of low compatibility with A.
A are polystyrene and copolymers of styrene which incorporate at least 80% by weight of styrene in the polymer. Examples of comonomers are xcex1-methylstyrene, ring-halogenated styrenes, ring-alkylated styrenes, acrylonitrile, (meth)acrylates of alcohols having from 1 to 8 carbon atoms, N-vinyl compounds, such as vinylcarbazole, or else small amounts of compounds which have two polymerizable double bonds, for example butadiene, divinylbenzene or butanediol diacrylate. Polystyrene is preferred.
The mixture of polymers A+B comprises from 1 to 50% by weight, preferably from 2 to 35% by weight, of a polymer B of low compatibility with A. The preferred amount is from 3 to 10% by weight if polymer B is polypropylene, and for polymethyl methacrylate it is from 10 to 25% by weight.
The incompatibility of B with A is determined as follows:
The glass transition temperature of the mixture of polymers is measured by DSC. If the glass transition points of the individual components are detectable separately in the DSC curve and have not been markedly shifted compared with the glass transition points of the individual components when pure, incompatibility is present. In case of doubt, the incompatibility of A with B can also be determined by electron microscopy, which if incompatibility is present shows two distinct phases if appropriate contrasting is used, phase A being the coherent phase.
Suitable polymers B are polyethylene, polypropylene, ethylene-vinyl acetate copolymer, polyurethane, polyamides, polycarbonate, polyether sulfones, polyoxymethylene, polyvinyl chloride and polyimide. Preference is given to polymethyl acrylate and polypropylene.
Another aspect of the present invention is that of open-cell moldable foams with increased heat resistance. These are obtained if the mixture of polymers A+B also comprises from 5 to 50% by weight, preferably from 10 to 30% by weight, based on A+B, of a thermoplastic C with a glass transition temperature above 140xc2x0 C., preferably above 180xc2x0 C. Examples of suitable thermoplastics C are styrene-maleic anhydride copolymers and polyphenylene sulfide, and preference is given to polyphenylene ethers. Adding the thermoplastic C raises the glass transition temperature of the thermoplastic matrix A+C to above 105xc2x0 C., with the result that the foam does not soften, e.g. when vacuum panels are foamed into polyurethane.
The starting material for producing the novel open-cell moldable foams is pellets comprising blowing agent. To prepare these, the polymers A, B and, if desired, C are first mixed in the melt, with conventional additives.
Conventional additives and/or auxiliaries which may be added to the thermoplastic matrix are conventional amounts of stabilizers, dyes, fillers, flame retardants and/or nucleating agents, preferably from 1 to 10% by weight, based on A+B+C, of an infrared absorber, e.g. aluminum powder or titanium dioxide powder, or of particulate carbon, in particular graphite powder, further increasing the open-cell content.
The pellets obtained after mixing are then impregnated with volatile blowing agents, using conventional processes, preferably in aqueous suspension at from 100 to 150xc2x0 C. and at a pressure of from 3 to 15 bar. Suitable blowing agents are saturated aliphatic C4-C7 hydrocarbons, in particular pentane.
In principle it is also possible for pellets A which comprise blowing agent to be mixed in the extruder with pellets B, and, where appropriate, C; alternatively, blowing agent may be introduced during the mixing of A, B, and, where appropriate, C. In these processes, the melt extrudate from the extruder is then usefully subjected, while under pressure, to underwater die-face cutting for pelletization.
The pellets comprising blowing agents are then prefoamed, using steam at temperatures of from 60 to 160xc2x0 C., whereuponxe2x80x94due to the polymer B which has been added and has low compatibility with polystyrenexe2x80x94the cell membranes break open to give an open-cell moldable foam. The density of the moldable foam can be adjusted as desired by repeated foaming. Finally, the moldable foam is sintered, using steam or hot air, in non-gas-tight molds, to give foam moldings. According to the invention, these have an open-cell content of more than 80%, preferably more than 90%, in particular at least 95%. If the open-cell content is too low the molding cannot be adequately evacuated. Its cells then hold too much gas, and thermal conductivity is not lowered sufficiently.
The novel open-cell molded foams may be evacuated to give vacuum moldings. These are used as vacuum panels in low-temperature insulation systems, e.g. in refrigerators, freezers and low-temperature transportation systems.